Direct measurement of phytoplankton growth rate in a eutrophic river reveals phytoplankton response to altered flow regimes

Background/Question/Methods

Agricultural production in arid regions results in large-scale modifications of landscape and hydrology, causing disruption of natural ecosystem function. Drainage from irrigated agriculture results in nutrient enrichment and the eutrophication of impacted rivers. In the Central Valley of California, the most productive arid farming region in the world, eutrophication of the San Joaquin River results in the export of phytoplankton biomass to the estuary, which creates regional anoxic conditions, negatively impacting fisheries. Fundamental information concerning factors influencing phytoplankton growth rates and yields is needed to understand, and ultimately control, eutrophication in this ecosystem.

In this study we tested the hypothesis that alteration in regulated river flows, planned as part of various restoration activities, would alter phytoplankton growth rates and yields and therefore exports of phytoplankton from the river to the estuary. Under experiments directed at predicting unintended outcomes from future restoration activities, river flow regimes were modified and phytoplankton response was measured using continuous chlorophyll monitoring and other methods. Direct measurement of phytoplankton growth kinetics were then used to inform and confirm predictions made by water quality and hydrologic models concerning the mass loading of phytoplankton to the estuary.

In this study, sondes were used to measure chlorophyll in the river and tributaries with high spatial and temporal resolution.  Chlorophyll measurements were complemented with grab sampling determination of nutrients and other factors of significance to phytoplankton metabolism. This data was combined with hydrologic data to allow in-situ measurement of growth rates and yields.

Results/Conclusions

Results indicate that that hydrologic factors are the major influences on observed phytoplankton growth rates and biomass yields (nutrients and light being typically in excess).    It was found that direct measurement of phytoplankton growth kinetics could be used to document changes in river eutrophication and water quality as planned ecosystem restoration activities are implemented. The results of these studies and the role of continuous data in the development of ecosystem restoration will be discussed.